1. Field of the Invention
This invention relates generally to precipitation sensors associated with monitoring the accumulation of precipitation upon window glass. More particularly, this invention relates to optical precipitation sensors used in automotive applications. Specifically, this invention relates to the optics used in automotive optical precipitation sensors and a method for their use.
2. Description of the Prior Art
It is desirable to free the driver, operating an automobile, from the distractions of manually performing certain functions associated with the operation of the automobile. Comfort and safety can be both served by automating these functions. Operation of the wipers for the windshield or other window glass of an automobile, is a function that has been automated.
Automating the operation of these wipers requires sensing the presence of water, or precipitation, upon the outer surfaces of the window glass. When water is sensed, a signal is generated, electronic circuitry processes the signal, and the wipers are automatically deployed to clear the water from the window glass surface. Several approaches have been taken toward this sensing of water on window glass. These have included sensing a change in conductivity or capacitance, at a sampling point upon the outer surface, when moisture is present. These have included acoustic effects produced by raindrops hitting the surface of the automobile (e.g. rain landing upon the window glass or some other portion of the vehicle). These approaches have also included various optical techniques.
Optical sensors operate on the principle that a light beam is diffused or deflected from its normal path by the presence of water on the outer surface of the window. The systems that use optical sensors have the distinct advantage that they are sensing the same or similar phenomenon, which gives rise to the need for wiper operation, that being the disruption of the light transmissibility of the window glass caused by water residing on the outer surface.
Generally, a beam of light, in the infrared or near infrared ranges, is emitted into the window glass, from inside of the automobile, and at an angle giving rise to total reflection at the outer surface. A photoelectric device, such as a photodiode or a phototransistor, then receives the reflected light and produces a representative electrical signal. The light received at the photoelectric device has certain characteristics when the outer surface is dry. The characteristics are altered when water is present on the outer surface, at the point where the light beam comes into contact with the outer surface. Since water has a refractive index close to that of glass, its presence causes a substantial portion of the light, which would otherwise be reflected to the receiver, to dissipate. This change in characteristics results in commensurate change in the electrical signal produced by the photoelectric device. The signal is processed by electronic circuitry to control the operation of the wipers.
A recent approach disclosed in U.S. Pat. No. 5,661,303 to Teder, for producing an optical precipitation sensor, includes the use of emission lenses to collimate infrared light emitted from multiple Light Emitting Diodes (LED) and to direct the light upon the outer surface of the window glass at angles giving rise to total reflection. Receiption lenses are then used to direct and focus the reflected emitted light upon receivers.
Another recent approach is disclosed in Czech Republic Patent numbered CZ 285,291 B6, to Lan et al., uses a rotational parabolic mirror to collimate and direct near infrared light from multiple LED's upon the outer surface at an angle giving rise to total reflection. The reflected emitted light is then directed and focused upon a receiver by another rotational parabolic mirror.
An issue that arises in connection with the use of optical sensors, for precipitation detection, is desensitization of the photoelectric device of the receiver, by ambient light. Bright ambient light, such as sunlight, impinging upon the photoelectric device of the receiver, causes the device to become relatively insensitive to the emitted light transmitted to the receiver. If enough ambient light is impinging upon the receiver, the signal produced by the receiver may not be adequately different, in response to the presence of water on the outer surface, to be useable by the electronics to reliably control the wipers.
The approach using lenses, of the '303 patent, apparently includes opaque members proximate and lateral to the optical axes of the reception lenses to block a portion of the ambient light reaching the receivers. The '291 patent does not discuss nor depict any means for blocking ambient light from reaching the receiver.
The U.S. Pat. No. 4,798,956 to Hochstein employed two methods toward overcoming the ambient light problem. For the first method, the receiver was placed at the bottom of a black tube to limit the number of directions from which ambient light could successfully reach the receiver. The use of infrared emitters was central to the second method employed. The '956 patent stated that infrared was used to compensate for ambient light. It indicated that commercially available infrared emitters emitted peak energy at 940 nm, in contrast to solar radiant energy peaking at approximately 500 nm. A filter was then placed in the tube between the opening of the tube and the receiver which passed the infrared light but rejected light of wavelengths shorter than infrared, including the peak solar wavelength of 500 nm.
Apparently, none of the approaches disclosed adequately protect the receiver from ambient light to ensure proper sensing of water on an outer surface of a window glass, in all light conditions expected to be encountered by a precipitation sensor.
Additionally, the advent of solar or thermal glass, for automotive applications, creates new challenges for the optical precipitation sensor designer. Solar glass includes additives to filter infrared and near infrared light from passing through the glass. Such glass protects the interior of the automobile from heating and other deleterious effects of this wavelength of light. However, it also substantially inhibits the infrared light of the emitter from reaching the receiver. It has been found that at least some infrared optical precipitation sensors are unusable in conjunction with such glass. The problem of ambient light rejection, evident in prior art designs, is exacerbated when the use of infrared emitters is no longer a viable option.
Accordingly, there remains the need for an optical precipitation sensor exhibiting improved ambient light rejection particularly when used in conjunction with solar or thermal glass.